1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a shape of a tire, and it relates to an apparatus for measuring a cross-sectional shape of a tread or of a sidewall of the tire with a tire installed on a vehicle.
2. Description of the Prior Art
It has been important to take hold of the state of tire wear and the progress of the wear has been taken hold of by depth gauge by measuring the depth of a groove formed on a tread. When one wants to know the cross-sectional shape of the tread as well as the depth of the groove, such a method has been employed as making a plaster cast of the tire tread and measuring the cross-sectional shape of the hardened plaster. This method has enabled us to take hold of the state of the tire wear in more detail including the state of uneven wear. However, since it takes time for the plaster to harden, the state could not be measured efficiently by the above mentioned method.
In order to shorten the time for measurement, apparatus that measures the shape of the tread by scanning an optical sensor by laser light in the tire width direction has been known (Patent Document 1: Examined Japanese Patent No. 3581876, pages 5 to 7, FIGS. 1 and 2). As shown in FIG. 1, in this apparatus, a housing 101 provided with an optical sensor 102 that can scan in the tire width direction contacts with a tread of a tire T, a bracket 103 fixed at one end of the housing 101 contacts with a side-wall of the tire T, a person who measures holds a handle 105 by hand, and the housing 101 is fixed. By scanning the optical sensor 102 in the width direction of the tire T and by measuring the distance to the tread, the shape of said tread is measured.
However, in the measuring apparatus disclosed in the patent document 1, as shown in FIG. 2, sometimes fixation is made in a state of positional deviation and not parallel to the rotational axis of the tire T. This tendency gets remarkable particularly when any of the shoulder portions of the tire T is in a state of uneven wear. Thus, measurement precision was degraded sometimes or measurement could not be made since the laser light 104 from the optical sensor 102 failed to reach the groove bottom.
Since the housing 101 contacts with the tread, raindrops or foreign materials adhere to or invade in the measuring apparatus sometimes causing the apparatus to fail. Further, since a person who measures holds the handle 105 by hand, due to shaking, measuring precision is sometimes degraded.
In the state where the housing 101 contacts with the tread, the precise position to be scanned by the laser light 104 of the optical sensor 102 becomes unknown. Therefore, when scanned by the laser light 104 along with the transverse groove, the shape of the tread of the portion cannot be measured, which in turns requires the positional change of the housing 101 to be measured again, causing to take more measuring time.
Therefore, the object of the present invention is to eliminate any factors that would degrade measuring precision or cause measuring incapability and to provide an apparatus capable of measuring the cross-sectional shape of the tire installed on a vehicle.